Creating an Internal network switch

# Create the internal switch with a name of VM-Internal-Switch
PS C:\WINDOWS\system32> New-VMSwitch -Name "VM-Internal-Switch" -SwitchType Internal
# Verify that the internal switch was created
PS C:\WINDOWS\system32> Get-VMSwitch
# Get the ifIndex of the newly created internal switch, usually named 'vEthernet (name)'
PS C:\WINDOWS\system32> Get-NetAdapter
# Set the IP address of the internal switch, noting the ifIndex retrieved from the previous command.
# In this example, the network address of the internal switch is 192.168.56.0/24, and the ifIndex is 50.
PS C:\WINDOWS\system32> New-NetIPAddress -IPAddress 192.168.56.1 -PrefixLength 24 -InterfaceIndex 50
# The VMs using the internal switch must use static IP addresses, such as 192.168.56.2/24.
# Support for DHCP in internal switches is not included in current Windows versions (as of Version 1703, OS Build 15063).

Now, by inspecting get-netadapter you should see a new Internal switch named VM-Internal-Switch, or whatever else you chose to name it.

Caveat emptor: For some reason I haven't been able to figure out yet, on boot the link doesn't come up, nor does the default route appear. You need to enable the link manually on boot, or through /etc/rc.local or something. Enabling the link and adding the route: